Science —

Examining the increased incidence of autism

Autism rates have been rising steadily over the past two decades, causing much …

Rates of autism, a developmental disorder that affects communication and social skills, are on the rise. Earlier in the 20th century, the incidence of the disorder was around four or five cases per 10,000 children; currently, it's more than an order of magnitude greater, being closer to 80 per 10,000. This precipitous rise is obviously quite worrisome, more so since we're no closer to knowing the reason for the increase.

Autism is usually diagnosed in early childhood, prior to the age of three. There's been a lot of (very ill-founded) controversy over a link between vaccination and autism, since diagnosis often occurs around the same time as childhood vaccinations. That link, suggested by a UK clinician, possibly working in concert with lawyers attempting to sue vaccine makers, has been thoroughly discredited by the scientific community, but it has made an impression on the public, as childhood diseases such as mumps, measles, and rubella have wreaked havoc in communities where vaccination rates dropped.

In looking for answers as to the increased incidence, fingers have been pointed at the methods of diagnosis, with the idea being that a more vigilant medical community, in conjunction with an expanded spectrum of autism diagnosis criteria, has resulted in greater numbers of cases. A pair of researchers from UC Davis and the Medical Investigations of Neurodevelopmental Disorders Institute in Sacramento have attempted to determine whether increased diagnosis is at fault, and have published the results in the journal Epidemiology.

Using data gathered by California's Department of Developmental Services (DDS), they examined a number of potential causes. Data gathered from 1990-2006 showed that the incidence of autism has been rising steadily, with a cumulative incidence (per 10,000 births) of 8.9 in the 1990 cohort, 22.2 in the 1994 cohort, rising to 40.4 for the 1998 cohort.

The first possible cause they rule out is that of changes to diagnosis criteria. Although the Diagnostic and Statistical Manual of Mental Disorders (commonly referred to as the DSM) was revised in the late 1980s and a new edition published in 1994, changes in the criteria for autism could only explain a 2.2-fold increase in rates, far below what has been observed. Early diagnosis was also not sufficient to explain the increase; although 12 percent more cases were diagnosed in children under the age of five in the 1996 group compared to 1990, there was no concurrent decrease in diagnosis of children aged between five and nine in the latter sample, which would be expected if we were simply detecting the condition earlier in the same children. Migration was also ruled out as a factor.

Unfortunately, the study doesn't really bring us much closer to identifying a reason for the increase in autism rates, but it does rule out some of the more common suppositions. Evidently there's still a lot of work left to do.